The discovery of ceramic compositions having superconducting properties is recent. Originally, superconductivity was observed in mercury at 4 K by the Dutch scientist, Heike Onnes in 1911. The term, superconductivity, refers to the property wherein a normally resistive conductor abruptly loses all resistance to electrical flow at a specific temperature, called the critical temperature, T.sub.c. At this point, the resistivity of the normal conductor becomes zero, i.e. the material becomes superconducting. In more recent times, Ogg (1946) studied superconductivity in quenched metal-ammonia solutions and proposed that superconductivity arose because of mobile electron-pairs. About 1973, it was determined that certain niobium metal alloys, i.e. Nb-Sn, exhibited superconductivity when cooled to liquid helium (4 K) temperatures. Later results raised this temperature as high as 23 K (-250.degree. C.). Until recently, it was believed that superconductivity above this temperature was not possible because of the theoretical work of Bardeen, Cooper and Schieffer (BCS theory-1946) which predicted such a limit. Several theoretical proposals were presented in the 1970's, suggesting that the critical temperature for superconductivity could be increased. However, the lack of any discoveries of superconductivity above 23 K solidified the belief that indeed this temperature could not be exceeded. Thus, when Bednorz and Muller announced, in November 1987, the discovery of a new ceramic superconducting compound based on lanthanum, barium, and copper oxides with a critical temperature for superconductivity close to 35 K., (G. Bednorz and A. Muller, Z. Phys., B64 189 (1986)), the declaration was greeted with considerable skepticism. Nevertheless, by the following month, the critical temperature, T.sub.c, for the onset of superconductivity was raised to nearly 80 K by C. W. Chu and coworkers (M. K. Wu, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett. 58 908 (1987)). This was achieved by changing the composition to yttrium barium copper oxide, approximated by the formula: EQU Y.sub.1.0 Ba.sub.1.8 Cu.sub.3.0 O.sub.6.3
This formula, determined experimentally, is not exactly stoichiometric. It is believed that this lack of specific nonstoichiometry contributes most to the onset of superconductivity. The mechanism of superconductivity in such oxide-based ceramic materials is not at all well understood. Ogg's original contribution suggested that superconductivity arose in quenched metal-ammonia solutions because of mobile electron pairs. The concept accepted at present is similar (the BCS theory), and suggests that if a mobile electron propagates through a lattice structure, it will normally interact with the bound electrons of the lattice because of differences in electron quantum spin number, and interact with the quantized vibrations of the lattice as well (vibronic coupling). However, if two such electrons form a pair which are bound through opposite spin-pairing (Cooper pairs), then no quantum interaction of the bound pairs can occur with the electrons of the lattice (which still have an electronic moment), or by vibronic coupling. That the BCS theory has some validity is shown by the following consideration. The so-called 1:2:3 compound, composed of Y--Ba--C--O atoms, is prepared by the solid state reaction of the requisite oxides, vis: EQU Y.sub.2 O.sub.3 +2BaO+3CuO=2YBa.sub.2 Cu.sub.3 O.sub.6.5.
It is now established (C. N. Rao et al, Nature, 327 185 (1987)) that high T.sub.c superconductivity in the Y--Ba--Cu--O system originates from a compound of stoichiometry: YBa.sub.2 Cu.sub.3 O.sub.7- , where " " is a value less than 1.0. This compound has the structure of the ideal perovskite, YBa.sub.2 CU.sub.3 O.sub.9. Thus, the superconductor YBa.sub.2 Cu.sub.3 O.sub.7- has about 25% fewer oxygen atoms present in the lattice as compared to the idealized cubic perovskite structure. This massive oxygen deficiency means that instead of the conventional three-dimensional crystalline cubic-stacking array of the perovskite, a unique layered structure results. A loss of even more oxygen atoms in this structure gives rise to the semiconductor, YBa.sub.2 Cu.sub.3 O.sub.6. The chain of copper atoms associated with a chain of oxygen atoms is believed to be the key to superconducting behavior. Yet the above description is an idealized one and the actual distinct structural conformation has not yet been delineated. Note that there appear to be extra oxygen atoms in the superconducting unit cell, compared to that of the semiconductor.
To date, most of the high-T.sub.c superconducting ceramic compositions announced to date are based on cuprate compounds having CuO.sub.2 layers as part of the structure. Some of these have included:
______________________________________ Bismuth Strontium Calcium Copper Oxide Bi.sub.2 Sr.sub.3-x Ca.sub.x Cu.sub.2 O.sub.8 + y T.sub.c = 114 lK. Thallium Calcium (Barium) Copper Oxide Tl Ba.sub.2 Ca Cu.sub.2 O.sub.7 Tl Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 Tl Ba.sub.2 Ca.sub.3 Cu.sub.4 O.sub.11 Tl Ba.sub.2 Ca.sub.4 Cu.sub.5 O.sub.13 T.sub.c = 120 lK. Lead Strontium Lanthanide Copper Oxide Pb.sub.2 Sr.sub.2 (Nd.sub.0.76 Sr.sub.0.24)Cu.sub.3 O.sub.8+x T.sub.c = 77 lK. ______________________________________
In the last compound given, the CuO.sub.2 -sheets are present but there is also a PbO-Cu-OPb sandwich as well, not observed in ceramic superconductors heretofore. The copper ions in this sandwich are monovalent and each is coordinated, above and below, to two oxygen atoms in the PbO layers. The copper atoms in the CuO.sub.2 sheets have an average valence of about 2.25, which is consistent with previously known cuprate compounds, given above. However, the presence of Cu.sup.+ atoms lowers the average valence of copper ions in the new structure to below 2.0, which is atypical. Indeed, preparation conditions needed to prepare these compounds includes a mildly reducing atmosphere so as not to oxidize Pb.sup.2+ to Pb.sup.4+.
There have also been some compositions announced, based on a copper-free composition, vis: EQU BAO--K.sub.2 O--Bi.sub.2 O.sub.3
This compound is said to become superconducting at about 30 K. While copper-oxide superconductors exhibit layered structures that carry current efficiently only along certain planes, this new material is a three-dimensional network of bismuth and oxygen with properties that are much less sensitive to crystallographic direction. It is hoped that compositions will be discovered in this system with temperature properties that rival those of copper-bearing compounds.
Superconducting compositions have been traditionally prepared by calcining carefully formulated mixtures of oxides. For example, to prepare the YBa.sub.2 Cu.sub.3 O.sub.7- superconducting phase, one weighs out 0.5 mol of Y.sub.2 O.sub.3, 2.0 mol of BaO, 3.0 mol cf CuO, and mixes them thoroughly. The mixture is then calcined at elevated temperature, in an oxidizing atmosphere, whereupon the oxides undergo solid state reaction to form a single phase with superconducting properties at 78 K. Alternately, one can choose compounds which decompose to form oxides which react to form the desired phase, when heated to elevated temperature.
Once the powder has been prepared, it can be handled by conventional means and processed to desirable forms. One such method employs a slurry of powder and methanol. By casting a uniform film on a suitable substrate such as sapphire, one can dry it, calcine it, and obtain a dense, uniform layer. However, a reannealing step in oxygen atmosphere is usually required to restore the critical oxide stoichiometry required for superconductivity.
There have been a number of methods used to prepare these ceramic superconducting compounds in useful form. One method deposits thin layers of the appropriate metal oxides in specific order on a silicon substrate by electron-beam evaporation. Another approach to preparation of superconducting films of Bi.sub.2 Sr.sub.3-x Ca.sub.x Cu.sub.2 O.sub.8+y has been to employ compounds which are volatile and to cause them to decompose on a hot surface in a partial vacuum. This method, known as vapor phase epitaxy, is capable of producing a superconductive monocrystalline film, using halogen compounds (or others) as the source materials, provided that suitable annealing in an oxygen atmosphere is carried out. Still another method for preparing superconductors in useful form has been the formation of a bar of the ceramic composition. Said bar was then heated on a pedestal by a LASER until it melted, a seed crystal was added, and a fiber was drawn at a controlled rate. The prototype wire was able to carry 30,000 A/cm.sup.2 at 4 K. before it failed, but was subject to the shortcomings of all ceramic fibers, namely flexibility and ductility.
Superconductive bolometers have been under investigation since 1946 to develop ultra-sensitive infrared detectors. (D. H. Andrews, R. M. Milton, and W. DeSorbo, "A Fast Superconducting Bolometer", J. Opt. Soc. Amer., 36(9), 518-524, (1946)). The sensitive element of such detectors is a metallic superconducting film evaporated on a substrate such as mica, sapphire, or quartz. The film temperature is controlled so it is in the transition region as shown in the accompanying FIG. 1. Incident infrared radiation causes the film temperature to rise, which results in an increase of the film resistance. The incident radiation power can be determined by measuring the resistance increase as long as the transition region characteristics are maintained. Accurate control of the film temperature is required prior to detection so that temperature fluctuations are small compared to signal produced by the temperature rise. In an alternative method, the film temperature is maintained just below the transition temperature (K. Weiser, U. Strom, S. A. Wolf, and D. U. Gubser, "Use of Granular NbN as a Superconducting Bolometer," J. Appl. Phys. 52, 1888-9, 1981). Incident radiation then causes the temperature of the film to rise above the transition temperature pf the superconductor, thus causing the film to become partially normal in conductivity. The film resistance is measured to determine the radiation flux.